The present invention relates generally to compositions having a nickel aluminide base and having a desirable stoichiometry which permits incorporation of greater quantities of ductilizing additives. More specifically, it relates to a rapidly solidified tri-nickel aluminide base alloy having improved ductilization based on boron doping and improved stoichiometry which enhances such doping.
It is known that polycrystalline tri-nickel aluminide castings exhibit properties of extreme brittleness, low strength and poor ductility at room temperature.
The single crystal tri-nickel aluminide in certain orientations does display a favorable combination of properties at room temperature including significant ductility. However, the polycrystalline material which is conventionally formed by known processes does not display the desirable properties of the single crystal material and although potentially useful as a high temperature structural material, has not found extensive use in this application because of the poor properties of the materials at room temperature.
It is known that nickel aluminide has good physical properties at temperatures above 1000.degree. F. and could be employed, for example, in jet engines as component parts for operating at higher temperatures. However, if the aluminide does not have favorable properties at room temperature and below, the part formed of this material may break when subjected to stress at the lower temperatures at which the part must be maintained prior to starting the engine and prior to operating the engine at the higher temperatures.
Alloys having a tri-nickel aluminide base are among the group of alloys known as heat-resisting alloys or superalloys. These alloys are intended for very high temperature service where relatively high stresses including tensile, thermal, vibratory and shock stresses, are encountered and where oxidation resistance is frequently required.
What has been sought in the field of superalloys is an alloy composition which displays favorable stress resistant properties not only at the elevated temperatures at which it may be used as, for example, in a jet engine but also a practical, desirable and useful set of properties at the lower temperatures to which the engine is subjected in storage and mounting and starting operations. For example, it is well known that an engine may be subjected to severe sub-freezing temperatures while standing on a field or runway prior to starting the engine.
Significant efforts have been made toward producing a tri-nickel aluminide and similar superalloys which may be useful over such a wide range of temperatures and which may be adapted to withstand stress to which the articles made from the material may be subjected in normal operations over a wide temperature range. For example, copending application Ser. No. 444,932, filed Nov. 29, 1982, now U.S. Pat. No. 4,478,791, and assigned to the same assignee as the subject application teaches a method by which a significant measure of ductility can be imparted to a rapidly solidified tri-nickel aluminide base metal at room temperature by addition of a small percentage of boron to overcome the brittleness of this material.
Also, copending application of the same inventors as the subject application, Ser. Nos. 647,327, 647,326, and 647,328, respectively, filed Sept. 24, 1984 teaches methods by which the composition and methods of the U.S. Pat. No. 4,478,791 may be further improved. These applications are incorporated herein by reference.
For the unmodified binary intermetallic, there are many reports in the literature of a strong dependence of strength and hardness on compositional deviations from stoichiometry. E. M. Grala in "Mechanical Properties of Intermetallic Compounds", Ed. J. H. Westbrook, John Wiley, New York (1960) p. 358, found a significant improvement in the room temperature yield and tensile strength in going from the stoichiometric compound to an aluminum-rich alloy. Using hot hardness testing on a wider range of aluminum compositions, Guard and Westbrook found that at low homologous temperatures, the hardness reached a minimum near the stoichiometric composition, while at high homologous temperature the hardness peaked at the 3:1 Ni:Al ratio. Met. Trans. 215 (1959) 807. Compression tests conducted by Lopez and Hancock confirmed these trends and also showed that the effect is much stronger for Al-rich deviations than for Ni-rich deviations from stoichiometry. Phys. Stat. Sol. A2 (1970) 469. A review by Rawlings and Staton-Bevan concluded that in comparison with Ni-rich stoichiometric deviations, Al-rich deviations increase not only the ambient temperature flow stress to a greater extent, but also that the yield stress-temperature gradient is greater. J. Mat. Sci. 10 (1975) 505 . Extensive studies by Aoki and Izumi report similar trends. Phys. Stat. Sol. A32 (1975) 657 and Phys. Stat. Sol. A38 (1976) 587. Similar studies by Noguchi, Oya and Suzuki also reported similar trends. Met. Trans. 12A (1981) 1647.
More recently, an article by C. T. Liu, C. L. White, C. C. Koch and E. H. Lee appearing in the "Proceedings of the Electrochemical Society on High Temperature Materials", ed. Marvin Cubicciotti, Vol. 83-7, Electrochemical Society, Inc. (1983) p.32, discloses that the boron induced ductilization of the same alloy system is successful only for aluminum lean Ni.sub.3 Al.
The subject application presents a further improvement in the nickel aluminide to which significant increased ductilization has been imparted by boron doping.